The 13th day of the month is more likely to be a Friday than any other day of the month.

This is because of the way the Gregorian calendar works. The Gregorian calendar has 400-year cycles has the same pattern of days. Thus, in the 400-year cycle there will be 688 Fridays that will be the 13th day of a month. Just slightly more than Sunday and Wednesday at 687. 

In numerology, the number twelve is considered the number of completeness. There are twelve months in a year, twelve hours on a clock, twelve tribes of Israel, and twelve apostles of Jesus to name a few examples. The number thirteen was considered irregular and countered the completeness of the number twelve. 

A superstition derived from the last supper is that if 13 people sit at a table, one of them will die. Thirteen was seen as an unlucky number and Friday was seen as an unlucky day. Many think that people combined the two and believed it to be incredibly unlucky. It wasn’t mentioned before the 19th century as the most unlucky day, though. 

However, the 13th day of the month is more likely to fall on a Friday than any other day of the month. In Spanish speaking countries, Tuesday the 13th is the unluckiest day. Famous people born on Friday the 13th are Mary-Kate and Ashley Olsen, Kate Walsh, and Darius Rucker. 

(Source)